All fixed versions of any Creative endeavor need a separate copyright for legal protection (short story, book, serial, movie, cartoon, audio performance, public reading, etc.). With that said, the act of fixing the work grants it (you) an automatic copyright. So it's debatable how important registration really is. However, if your IP blows up into a million dollar revenue stream, registration is a pretty good idea, otherwise...meh. Of course, even for the average IP, it certainly won't hurt, and it's not all the expensive or time consuming to do. Generally speaking though, the main reason for copyrights and trademarks is to protect the Creative from BS claims and litigation being filed against them, more so than to protect the IP itself from theft or piracy. It's a defensive measure.
There's two types of copyrights at work here:
The composition - or the IP. This grants the holder the right to collect revenue from unit sales (publishing rights/mechanical royalties).
And the recording performance itself. This grants the holder the right to collect performance royalties. You'll want to get a Transfer of these Rights if you're not the one doing the recording to simplify your life. I assume this is pretty standard in the industry.
Don't think of AI narration as AI, just think of it as narration. It's no different than if you used MS Word to narrate the work. This is common on YouTube, and the work is still protected-ish. But the important thing to think about here is what this copyright is protecting - (public) Performance Royalties. How often do we hear a recording of a book in the wild? Or samples being used? The most common transgressor here is, again, YouTube (or TikTok or Instagram) - a fan using the recording on their channel, better known as free advertising. So there's no reason to complain about that.
With that said, Bill may be correct, but not because of any AI legal concern. If the book is simply narrated (by computer or app) without any additional work, such as music or sound effects - there's not really anything to copyright per se on the performance side of things, because there are no performing rights to protect. It makes the copyright moot. Copyright litigation requires proof of both standing and harm. MS Word can prove neither.
This is a great article explaining copyrights for music, which is pretty much the same for audio books because they're both sound recordings of a composition.
https://lawyerdrummer.com/2020/10/the-2-copyrights-in-a-song/And for audio book sound recording:
https://www.sidebarsaturdays.com/2017/07/01/httpwp-mep7vddb-td/